1. Field of the Invention
The present invention relates to an apparatus and process for extruding granules. Preferably, the granules are formed from dry powder material.
2. Description of Related Art
The extruders which are currently used employ rotating single or double screws to transport a moistened and kneaded material to be granulated to a die having holes. The pressure of the feeding screw and the scraping action of the screw(s) forces the material to come out of the holes of the die in the form of cylindrical strands.
One of the earliest extruders was an axial extruder which has either a single screw or a double screw. A schematic representation of such an extruder is shown in FIG. 1 of the drawings accompanying this specification. Generally, this extruder uses a rotating screw driven by a motor to convey the material to be extruded to a die having holes. The die is aligned axially with the screw. The material to be extruded is fed through the die and forced to transfer forwardly to the front end of the extruder by means of the conveying screw. The inaterial is pressurized and compressed to extrude out of the holes of the die. Hence, in these types of extruders, the compaction of the material is very high.
An improved version of the above-described extruder was fabricated to overcome the problem of high pressure and low production capacity of small sized granules found in the earlier types of extruders. This improved version is known as a screw-type radial extruder as depicted in FIG. 2. Generally, the raw material which is to be extruded is conveyed and cornpressed by a conveying screw axially within the screw housing. The material is then forced radially outward through a semi cylindrical die by means of an extracting screw. In this case, the extruding pressure changes its direction from axial flow to radial flow. Hence, the impact on the material employing this type of extruder is lower than the previously described extruder. However, the internal pressure generated by the conveying screw cannot be fully employed for the extrusion of the material through the die because the material changes its direction from axial to radial. This again leads to lower production capacity resulting in making the device uneconomical.
Alternatively, a low pressure basket type extruder positioned vertically/horizontally may be employed. This low pressure basket type extruder is depicted in FIG. 3. Generally, the low pressure basket type extruder consists of a basket shape housing holding a spherical plate with multiple holes. In this type of extruder, when the material to be extruded is fed to ihe extruder the material encounters a distributing plate which distributes the material uniformly. The extrusion blades, which are mounted to the rotating axis of the extruder, push the material out of the holes of the die in the form of strands. In this type of extruder, no screw is provided. Hence, the extrusion pressure is minimized and eventually the granules are formed under low impact and pressure. The problem encountered in these types of extruders is that it can not extrude a powder that is not moistened with externally added water or other solvents.
The present invention is an extruder and granulation process for producing uniformly sized granules. The extruder contains a chamber having a chamber inlet and an apertured plate located at a bottom of the chamber. A spiral worm screw is positioned within the chamber for guiding a granulation material to the apertured plate. In a preferred embodiment, the granulation material is a dry powder material that is substantially free from water and/or solvent having a low softening point. The spiral worm screw, which includes a guiding slant terminating at a blunt edge at a distance from said apertured plate, is rotated within the chamber. When the granulation material is inserted into the chamber through the chamber inlet, the spiral worm screw is rotated. Upon rotation of the spiral worm screw, the blunt edge rubs the granulation material against the apertured plate resulting in an increase in pressure between the, granulation material and the apertured plate. This increase in pressure softens the granulation material transforming it to a semi-solid material. The semi-solid material is then gently extruded through the apertured plate using just the required pressure. The granules are hardened after extrusion through the apertured plate. The extruder may be used in a continuous manner to produce the desired granules.
The extruder and granules that are formed are preferred over the prior devices and compounds for a variety of reasons. When the granulation material is a dry powdered material substantially free from water and/or solvent, the material exiting the apertured plate is in the form of granules without the additional cutting step that is required in the prior art. However, additional cutting may be performed if the user requires a smaller granule size. Since additional water and/or solvents are not required in this granulation process, the granules that are formed do not need to be dried. However, drying of the granules may optionally be performed depending on the moisture content that is required. The rubbing action produced when the blunt edge pushes the granulation material against the apertured plate affects the water dispersibiltity of the granules and is preferred over previous compaction and impact granulation devices. Although a wide range of granules may be formed using the present extruder, the preferred diameter for the granules is 1-2 mm.
The present extruder is useful for granulation materials in all forms, especially dry powder material, which avoids the problems encountered in the prior extruders. ln;a preferred embodiment, the granules that are produced are water soluble or water dispersible granules having excellent water solubility and/or water dispersing quality for essentially environmentally friendly uses. Most preferably, the present extruder may be used for preparing granules of dry powdered, water soluble, low melting compounds and compositions, such as Acephate, Lambdacyhalothrin, Chloropropham, Metalaxyl, Bifenthrin, Devrinol, Chlorpyrifos, Endosulfan, Glyphosate, IPA salt, Varnidothion, Trichloron and the like without the problems encountered in prior devices.